1. Field of the Invention
The present invention relates to a transmission gear, and more particularly to a transmission gear for transmitting two independent rotary motions from a driving mechanism to a working mechanism.
2. Description of the Related Art
There have so far been proposed a wide variety of transmission gears for transmitting a mechanical motion from certain devices to other devices. The transmission gears are generally utilized as change speed gears or speed reduction gears for converting a high speed rotary motion to a low speed rotary motion of large torque. The gears of these types include a so-called harmonic drive gear comprising input and output shaft members, and a flexible metal member intervening between the input and output shaft members. Such a gear mechanism is disclosed in, for example, Japanese Patent Laid-open Publication No. 1-216141 as comprising an input shaft member eccentrically formed with a cam portion and receiving the rotary motion from a power source such as an electric motor, and an output shaft member coaxially aligned with the input shaft member and connected to a working mechanism. The gear mechanism further comprises an externally toothed ring member rotatably supported by the cam portion of the input shaft member, an internally toothed ring member held at a standstill and in mesh with the externally toothed ring member, and a flexible member connected at one end thereof to the externally toothed ring member and at the other end thereof to the output shaft member. Since the internally toothed ring member is held at a standstill, the rotation of the cam portion of the input shaft member causes the externally toothed ring member to revolve slowly around the input shaft member in accordance with a difference in tooth number between the internally toothed ring member and the externally toothed ring member. The revolution of the externally toothed ring member is transmitted to the output shaft member through the flexible member. At this time, the flexible member functions as means for canceling the eccentric rotation between the externally toothed ring member and the output shaft member.
In the transmission gear thus constructed, if the driving mechanism side and the working mechanism side of the transmission gear are desired to be disconnected in spatial communication from each other, there is a necessity for the transmission gear to be equipped with spatial sealing means for disconnecting the driving mechanism side and the working mechanism side of the transmission gear from each other. The transmission gear is provided with the sealing means by adopting a magnetic fluid sealing system, a direct drive system, a magnetic coupling system or the like. If the transmission gear partially forms an apparatus for manufacturing a semiconductor devices, the working mechanism side of the transmission gear is frequently required to be pumped out to make a vacuum. At the same time, the working mechanism side of the transmission gear is required to be prevented from discharging gas for the purpose of obtaining a complete vacuum. For the this reason, the transmission gear is previously baked at high temperature so as to discharge the gas. The foregoing sealing means, however, cannot be baked at fully high temperature and, as a result, the complete vacuum cannot be obtained.
In order to eliminate the drawback, there has been proposed another transmission gear comprising a cup-shaped flexible member and disclosed in, for example, Japanese Patent Laid-open Publication No. 2-186151. The flexible member serves as both of the externally toothed ring member and the flexible member disclosed in the former Japanese Patent Laid-open Publication No. 1-216141. In the transmission gear disclosed in the latter Publication, the cup-shaped flexible member is fixedly supported by a support member held at a standstill, while the internally toothed gear is rotatable with respect to the support member and connected to the output shaft member.
If, however, a plurality of rotary motions are desired to be transmitted from the driving mechanism to the working mechanism, a drawback is encountered in prior-art transmission gears of the above described nature in that it is necessary for the transmission gear to include a plurality of flexible members respectively intervening between the input shaft members and the output shaft members and accordingly that the transmission gear increases in cost, complexity of construction and occupation space.